Segment Information (Net Premiums Earned By Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Total net premiums earned	$ 646,376	$ 537,247	$ 1,214,924	$ 1,067,532
Operating Segments [Member] | North American Insurance [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	328,541	265,800	641,511	531,251
Operating Segments [Member] | North American Insurance [Member] | General Casualty [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	109,218	78,565	214,143	154,865
Operating Segments [Member] | North American Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	84,411	64,259	163,327	128,531
Operating Segments [Member] | North American Insurance [Member] | Healthcare [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	38,396	43,323	76,437	85,817
Operating Segments [Member] | North American Insurance [Member] | Programs [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	36,710	28,708	70,857	65,478
Operating Segments [Member] | North American Insurance [Member] | General Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	27,140	30,539	55,140	59,176
Operating Segments [Member] | North American Insurance [Member] | Inland Marine [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	15,380	10,577	28,728	19,468
Operating Segments [Member] | North American Insurance [Member] | Environmental [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	7,273	4,774	14,095	9,360
Operating Segments [Member] | North American Insurance [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	10,013	5,055	18,784	8,556
Operating Segments [Member] | Global Markets Insurance [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	102,626	37,998	152,666	73,009
Operating Segments [Member] | Global Markets Insurance [Member] | General Casualty [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	21,714	3,208	27,626	5,901
Operating Segments [Member] | Global Markets Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	22,347	12,918	41,850	24,638
Operating Segments [Member] | Global Markets Insurance [Member] | Healthcare [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	3,800	2,950	7,406	5,452
Operating Segments [Member] | Global Markets Insurance [Member] | General Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	18,297	8,759	26,529	16,998
Operating Segments [Member] | Global Markets Insurance [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	4,177	0	4,179	0
Operating Segments [Member] | Global Markets Insurance [Member] | Motor [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	9,319	0	9,319	0
Operating Segments [Member] | Global Markets Insurance [Member] | Marine [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	8,680	710	10,931	918
Operating Segments [Member] | Global Markets Insurance [Member] | Aviation [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	5,351	4,733	11,304	10,370
Operating Segments [Member] | Global Markets Insurance [Member] | Construction [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	4,955	0	5,261	0
Operating Segments [Member] | Global Markets Insurance [Member] | Trade Credit [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	3,986	4,720	8,261	8,732
Operating Segments [Member] | Reinsurance [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	215,209	233,449	420,747	463,272
Operating Segments [Member] | Reinsurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	104,817	128,931	209,168	266,014
Operating Segments [Member] | Reinsurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	58,880	50,161	115,782	97,677
Operating Segments [Member] | Reinsurance [Member] | Specialty [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 51,512	$ 54,357	$ 95,797	$ 99,581